Level II scour analysis for Bridge 5 (MORRTH00060005) on Town Highway 6, crossing Bedell Brook, Morristown, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MORRTH00060005 on Town Highway 6 crossing Bedell Brook, Morristown, Vermont
(figures 1–8). A Level II study is a basic engineering analysis of the site, including a
quantitative analysis of stream stability and scour (U.S. Department of Transportation,
1993). Results of a Level I scour investigation also are included in Appendix E of this
report. A Level I investigation provides a qualitative geomorphic characterization of the
study site. Information on the bridge, gleaned from Vermont Agency of Transportation
(VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is
found in Appendix D.
The site is in the Green Mountain section of the New England physiographic province in
north-central Vermont. The 6.28-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of pasture, shrubs, and
brushland.
In the study area, Bedell Brook has a sinuous channel with a slope of approximately 0.01 ft/
ft, an average channel top width of 56 ft and an average bank height of 4 ft. The
predominant channel bed material is gravel with a median grain size (D₅₀) of 35.8 mm
(0.117 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
July 16, 1996, indicated that the reach was laterally unstable. There are wide point bars and
cut-banks with slipping bank material noted upstream and downstream of this site.
The Town Highway 6 crossing of Bedell Brook is a 44-ft-long, two-lane bridge consisting
of one 42-foot concrete T-beam span (Vermont Agency of Transportation, written
communication, October 26, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 45 degrees to the opening while the
opening-skew-to-roadway is zero degrees.
A scour hole up to 1.5 ft deeper than the mean thalweg depth was observed along the left
abutment and upstream and downstream left wingwalls during the Level I assessment. The
scour protection measure at this site was type-4 stone fill (less than 60 inches diameter) on
the left bank and left wingwall upstream, the left abutment and the downstream left
wingwall. Additional details describing conditions at the site are included in the Level II
Summary and Appendices D and E.
Scour depths and recommended rock rip-rap sizes were computed using the general
guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995).
Total scour at a highway crossing is comprised of three components: 1) long-term
streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction
in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and
abutments). Total scour is the sum of the three components. Equations are available to
compute depths for contraction and local scour and a summary of the results of these
computations follows.
Contraction scour for all modelled flows ranged from 1.1 to 2.0 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 3.9 to
8.6 feet. The worst-case abutment scour occurred at the 500-year event. Additional
information on scour depths and depths to armoring are included in the section titled “Scour
Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented
in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure
8. Scour depths were calculated assuming an infinite depth of erosive material and a
homogeneous particle-size distribution.
It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.